Synchronous Frequency Hopping Spread Spectrum Communications

ABSTRACT

Synchronous FHSS networks operating within mesh networks typically require a certain amount of network traffic to maintain time as well as for executing other functions, such as registration and neighbor discovery. The concepts presented in this disclosure provide a mesh network with enhanced communication capabilities without adding significant hardware or firmware costs to nodes within the network. The disclosed concept of using acquisition channels (frequencies) integrated within FHSS pseudo-random sequences speeds network responses to conditions like outage and restoration. Assignment of unique hop sequences by hop level or at time of manufacture can guarantee minimal network contention while minimizing system network traffic.

CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No.14/517,108 filed on Oct. 17, 2014, which claims the benefit under 35U.S. §119(e) of Provisional U.S. Patent Application No. 61/892,774,filed on Oct. 18, 2013, and entitled “SYNCHRONOUS FHSS METHOD FOR MESHNETWORKS,” the contents of both of which are incorporated by referenceherein in their entireties.

TECHNICAL FIELD

The present invention relates to communication systems and methods, andmore particularly, to systems, methods, and apparatus for improvedperformance in a wireless communications system.

BACKGROUND

Many utilities have begun to implement frequency hopping spread spectrum(FHSS) communications in a mesh network of metering devices. Some FHSSimplementations operate in a synchronous mode in which receiving devicesare substantially continuously synchronized with transmitting devices intime or frequency. Such synchronous implementations typically requirethe use of clock circuitry and require some amount of data traffic tomaintain time within the system. Other FHSS implementations operate in aself-synchronous mode in which receiving devices are not synchronized atall times with transmitting devices in time or frequency. In suchself-synchronous implementations, a receiving device scans a number offrequencies in the hop sequence in order to detect a preamble signal onone of the frequencies. Once detected, the receiving device will attemptto lock onto that channel, and the two devices will then begin hoppingin sequence. While self-synchronous systems do not require the same typeof clock circuitry as synchronous systems and avoid the communicationsoverhead required to maintain synchronization among nodes, such systemsdo typically require a long preamble to be transmitted in order toprovide sufficient opportunity for a receiver to scan and lock onto achannel. This itself may result in some data overhead.

The foregoing background discussion is intended solely to aid thereader. It is not intended to limit the innovations described herein.Thus, the foregoing discussion should not be taken to indicate that anyparticular element of a prior system is unsuitable for use with theinnovations described herein, nor is it intended to indicate that anyelement is essential in implementing the innovations described herein.The implementations and application of the innovations described hereinare defined by the appended claims.

SUMMARY

Disclosed herein are methods, systems, and apparatus for synchronousfrequency hopping spread spectrum communications in a mesh network ofutility meters. An embodiment of the present disclosure includes acommunication system that includes at least a first network device and asecond network device. Each network device has a transceiver. The firstnetwork device operates in accordance with a first frequency hoppingsequence. The first frequency hopping sequence includes a plurality ofdata channels and at least one acquisition channel. The second networkdevice operates in accordance with a second frequency hopping sequence.The second frequency hopping sequence includes a plurality of datachannels and the at least one acquisition channel. The second frequencyhopping sequence is different from the first frequency hopping sequence.The at least one acquisition channel of the first frequency hoppingsequence and the at least one acquisition channel of the secondfrequency hopping sequence are arranged in their respective frequencyhopping sequences such that when the second network device is tuned tothe at least one acquisition channel of the second frequency hoppingsequence, the first network device will be tuned to the at least oneacquisition channel of the first frequency hopping sequence, and thefrequency of the at least one acquisition channel in each of the firstand the second frequency hopping sequences is the same.

Additionally, another embodiment of the present disclosure includes amethod for communication. The method for communication includesassigning a first frequency hopping sequence to a first network device,wherein the first frequency hopping sequence includes a plurality ofdata channels and at least one acquisition channel; and assigning asecond frequency hopping sequence to a second network device, whereinthe second frequency hopping sequence includes a plurality of datachannels and the at least one acquisition channel. The second frequencyhopping sequence is different from the first frequency hopping sequence.The at least one acquisition channel of the first frequency hoppingsequence and the at least one acquisition channel of the secondfrequency hopping sequence are arranged in their respective frequencyhopping sequences such that when the second network device is tuned tothe at least one acquisition channel of the second frequency hoppingsequence, the first network device will be tuned to the at least oneacquisition channel of the first frequency hopping sequence, and thefrequency of the at least one acquisition channel in each of the firstand the second frequency hopping sequences is the same.

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the Description ofthe Invention section. This Summary is not intended to identify keyfeatures or essential features of the claimed subject matter, nor is itintended to be used to limit the scope of the claimed subject matter.Furthermore, the claimed subject matter is not constrained tolimitations that solve any or all disadvantages noted in any part ofthis disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding may be had from the following description,given by way of example in conjunction with the accompanying drawings,wherein:

FIG. 1 illustrates one example of a metering system in which thesystems, methods, and apparatus disclosed herein may be embodied.

FIG. 2A illustrates a block diagram of a data collector/utility meter,according to an aspect of the disclosure.

FIG. 2B illustrates a block diagram of a utility meter, according to anaspect of the disclosure.

FIG. 3 is a flowchart illustrating one embodiment of a method for timesynchronization, according to an aspect of the disclosure.

DESCRIPTION OF THE INVENTION

Disclosed herein are methods, systems, and apparatus for communicationsin a frequency hopping communications network for use in utilitymetering, and in particular, for synchronizing network devices (e.g.,utility meters) and broadcasting messages within such network. In thedisclosed system, different ones of the meters may communicate inaccordance with one of a plurality of different frequency hoppingsequences. In accordance with one aspect, each of the plurality ofdifferent frequency hopping sequences includes at least one acquisitionchannel (i.e., frequency), one which the device utilizing that sequencecan receive time synchronization messages, broadcasted messages, orother messages related to the network. Further in accordance with thisaspect, the plurality of different frequency hopping sequences isconfigured such that the acquisition channels in the sequences coincidein time. That is, when one network device is tuned to an acquisitionchannel in the frequency hopping sequence utilized by that device, eachof the other network devices within the network will also be tuned tothe same acquisition channel in their respective hop sequences—eventhough the other frequencies in their respective hop sequences may notbe the same. As described hereinafter, these acquisition channels may beused to facilitate synchronization of clock circuitry within each devicewith a common network time and to facilitate the transmission ofbroadcast messages to devices in the network.

FIG. 1 provides a diagram of an embodiment of a metering system 110 inwhich the methods, systems, and apparatus disclosed herein may beemployed. System 110 comprises a plurality of meters 114, which areoperable to sense and record consumption or usage of a service orcommodity such as, for example, electricity, water, or gas. Meters 114may be located at customer premises such as, for example, a home orplace of business. Meters 114 comprise circuitry for measuring theconsumption of the service or commodity being consumed at theirrespective locations and for generating data reflecting the consumption,as well as other data related thereto. Meters 114 may also comprisecircuitry for wirelessly transmitting data generated by the meter to aremote location. Meters 114 may further comprise circuitry for receivingdata, commands or instructions wirelessly as well. Meters that areoperable to both receive and transmit data may be referred to as“bi-directional” or “two-way” meters, while meters that are only capableof transmitting data may be referred to as “transmit-only” or “one-way”meters. In bi-directional meters, the circuitry for transmitting andreceiving may comprise a transceiver. In an illustrative embodiment,meters 114 may be, for example, electricity meters manufactured byElster Solutions, LLC and marketed under the tradename REX.

System 110 further comprises collectors 116. In some embodiments, acollector 116 may also be referred to as a gatekeeper. In oneembodiment, collectors 116 are also meters operable to detect and recordusage of a service or commodity such as, for example, electricity,water, or gas. In addition, collectors 116 are operable to send data toand receive data from meters 114. Thus, like the meters 114, thecollectors 116 may comprise both circuitry for measuring the consumptionof a service or commodity and for generating data reflecting theconsumption and circuitry for transmitting and receiving data. In oneembodiment, collector 116 and meters 114 communicate with and amongstone another using a frequency hopping communications technique, such as,for example, a frequency hopping spread spectrum (FHSS) technique.

A collector 116 and the meters 114 with which it communicates define asubnet/LAN 120 of system 110. As used herein, meters 114 and collectors116 may be referred to as “network devices” or “devices” in the subnet120. In each subnet/LAN 120, each meter transmits data related toconsumption of the commodity being metered at the meter's location. Thecollector 116 receives the data transmitted by each meter 114,effectively “collecting” it, and then periodically transmits the datafrom all of the meters in the subnet/LAN 120 to a data collection serveror head-end system 206. The data collection server 206 stores the datafor analysis and preparation of bills, for example. The data collectionserver 206 may be a specially programmed general purpose computingsystem and may communicate with collectors 116 via a network 112. Thenetwork 112 may comprise any form of network, including a wirelessnetwork or a fixed-wire network, such as a local area network (LAN), awide area network, the Internet, an intranet, a telephone network, suchas the public switched telephone network (PSTN), a Frequency HoppingSpread Spectrum (FHSS) radio network, an ISM mesh network, a Wi-Fi(802.11) network, a Wi-Max (802.16) network, a land line (POTS) network,or any combination of the above.

FIG. 2A is a block diagram illustrating further details of oneembodiment of a collector 116. Although certain components aredesignated and discussed with reference to FIG. 2A, it should beappreciated that the invention is not limited to such components. Infact, various other components typically found in an electronic metermay be a part of collector 116, but have not been shown in FIG. 2A forthe purposes of clarity and brevity. Also, the invention may use othercomponents to accomplish the operation of collector 116. The componentsthat are shown and the functionality described for collector 116 areprovided as examples, and are not meant to be exclusive of othercomponents or other functionality.

As shown in FIG. 2A, collector 116 may comprise metering circuitry 204that performs measurement of consumption of a service or commodity and aprocessor 205 that controls the overall operation of the meteringfunctions of the collector 116. The collector 116 may further comprise adisplay 210 for displaying information such as measured quantities andmeter status and a memory 212 for storing data. The collector 116further comprises wireless LAN communications circuitry 214 forcommunicating wirelessly with the meters 114 in a subnet/LAN and anetwork interface 208 for communication over the network 112. As furthershown, the collector 116 includes a clock circuit 203. The clock circuit203 for the collector 116 may run off an internal 12 MHz crystal and maybe adjusted from the central station on a daily basis (or more often).During outages, the clock circuit 203 may keep using a 32 kHz crystal.In an alternative embodiment, the collector 116 may use a 60 Hz linefrequency for additional timing accuracy adjustments.

In one embodiment, the metering circuitry 204, processor 205, display210 and memory 212 may be embodied in a commercially available meter,such as in an A3 ALPHA meter available from Elster Solutions, LLC. Inthat embodiment, the wireless LAN communications circuitry 214 may beimplemented by a LAN Option Board (e.g., a 900 MHz two-way radio)installed within the A3 ALPHA meter, and the network interface 208 maybe implemented by a WAN Option Board (e.g., a telephone modem) alsoinstalled within the A3 ALPHA meter. The WAN Option Board 208 routesmessages from network 112 (via interface port 202) to either the meterprocessor 205 or the LAN Option Board 214. LAN Option Board 214 may usea transceiver (not shown), for example a 900 MHz radio, to communicatedata to meters 114. Also, LAN Option Board 214 may have sufficientmemory to store data received from meters 114. This data may include,but is not limited to the following: current billing data (e.g., thepresent values stored and displayed by meters 114), previous billingperiod data, previous season data, and load profile data.

FIG. 2B is a block diagram of an exemplary embodiment of a meter 114that may operate in the system 110 of FIG. 1. As shown, the meter 114comprises metering circuitry 204′ for measuring the amount of a serviceor commodity that is consumed, a processor 205′ that controls theoverall functions of the meter, a display 210′ for displaying meter dataand status information, and a memory 212′ for storing data and programinstructions. The meter 114 further comprises wireless communicationscircuitry 214′ for transmitting and receiving data to/from other meters114 or a collector 116. The wireless communications circuitry 214′ maybe similar to or identical to the wireless communication circuitry 214in the collector 116 of FIG. 2A. The meter 114 also comprises a clockcircuit 203′ like the collector 116. The clock circuit 203′ may besimilar or identical to the clock circuit 203 used in the collector 116.

The collector 116 may be responsible for managing, processing androuting data communicated between the collector 116 and network 112 andbetween the collector 116 and meters 114. Collector 116 may continuallyor intermittently receive current data from meters 114 and store thedata in memory 212 or a database (not shown) in collector 116. Suchcurrent data may include but is not limited to the total kWh usage, theTime-Of-Use (TOU) kWh usage, peak kW demand, and other energyconsumption measurements and status information. Collector 116 also mayreceive and store previous billing and previous season data from meters114 and store the data in memory 212 or the database in collector 116.The database may be implemented as one or more tables of data within thecollector 116.

In an embodiment, the metering system 110 may be an Advanced MeteringInfrastructure (AMI) system which uses the ANSI C12.22 protocol forinterfacing with the network 112. It should be appreciated that otherprotocols may be used for the methods and systems for datacommunications defined herein, for example, ANSI C12.21 and ANSI 12.18.The protocol makes provisions for encrypting data to enable securecommunications, including confidentiality and data integrity, for thepurpose of interoperability between metering devices and the network.

In an embodiment, the LAN/subnet formed by a collector 116 and theplurality of meters 114 that communicate with it may operate to form awireless mesh network that implements FHSS techniques in the 900 MHz ISMband. It should be appreciated that the system and method disclosedherein may comply with Federal Communications Commission (FCC) part15.247 while providing mechanisms for devices (e.g., meters 114 andcollectors 116) to join, register, synchronize, and find neighborswithin a LAN/subnet.

In accordance with one aspect of the methods, systems, and apparatusdisclosed herein, in the metering system 110, each network device (e.g.,meters 114 and collectors 116) is assigned, and operates in accordancewith, one of a plurality N of different frequency hopping sequences.That is, the communications circuirty (i.e., transceiver) of the devicecommunicates in accordance with FHSS techniques and employs one of the Nfrequency hopping sequences. The hopping sequence assigned to aparticular device may be defined at the time of manufacture or assigneddepending on other factors, such as, for example, the particular hoplevel of the device (i.e., the number communication hops from thedevice, through one or more intermediate devices, to a collector). Inone embodiment, the devices may be equally distributed among the Nhopping sequences.

In accordance with another aspect, within a given frequency band, thetotal number of channels (numChan) in each hopping sequence may bedefined. And further, of the total number of channels (numChan), asubset of channels (m) may be defined as acquisition (ACQ) channels, andall other channels (numChan-m) may be defined as data channels. In oneembodiment, as a network device hops from one channel to the next in itsassigned frequency hopping sequence, the dwell time on each channel maybe long, for example, around 400 msec. In other embodiments, the dwelltime may be shorter or longer than 400 msec.

In one embodiment, devices in the FHSS network may communicate withinthe 902-928 MHz frequency band. In that embodiment, the following mayapply:

numChan=64 (64 total channels)

N=16 (16 hopping sequences)

m=4 (4 Acquisition Channels)

60 data channels

Dwell time per channel=400 msec

Hopping sequence repeats every 64 * 0.4=25.6 seconds

That is, in this embodiment, each network device is assigned to one of16 frequency hopping sequences. Each hopping sequence includes 64channels, of which, 4 are acquisition channels. The dwell time on eachchannel for each network device is 400 msec, resulting in a hoppingsequence repeating every 25.6 seconds. Table 1, below, illustrates thisembodiment, and includes 16 hopping sequences having four acquisitionchannels (4, 20, 36, and 52) each. These channels represent an offsetinto a pseudo-random channel sequence. At the four acquisition channels,each frequency hopping sequence is tuned to the same channel. In Table1, each hopping sequence is shown as a series of 64 channels, numbered 0through 63. For example, frequency hopping sequence “0” begins in time“0” on frequency “0,” followed by time period “1” on frequency “1”, timeperiod “2” on frequency “2”, and so on. Frequency hopping sequence “1”begins in time “0” on frequency “61,” followed by time period “1” onfrequency “62,” time period “2” on frequency “63,” time period “4” onfrequency “0” and so on. It is understood that each frequency (0, 1, 2,3, 4 . . . 63) constitutes a different frequency, or channel, within the902-928 MHz frequency band.

As illustrated in Table 1, in this example embodiment, the four ACQchannels in each frequency hopping sequence coincide at time periods 4,20, 36, and 52 in each sequence. Thus, when one network device is tunedto an acquisition channel in the frequency hopping sequence utilized bythat device, each of the other network devices within the network willalso be tuned to the same acquisition channel in their respective hopsequences—even though the other frequencies in their respective hopsequences may not be the same at any given time. As describedhereinafter, these acquisition channels may be used to facilitatesynchronization of clock circuitry within each device with a commonnetwork time and to facilitate the transmission of broadcast messages todevices in the network. While in this embodiment, numChan=64, N=16, andm=4, it is understood that in other embodiments, different numbers oftotal channels, different numbers of hop sequences, and differentnumbers of acquisition channels may be employed—either more or less ineach case. Moreover, it is understood that while in this exampleembodiment the channels are spread across the 902-928 MHz frequencyband, the techniques described herein may be employed across otherfrequency bands.

TABLE 1 Frequency Hopping Sequences 0 1 2 3 4 5 6 7 8 9 10 11 12 13 1415 Network Time Slot t (0) 0 61 58 55 51 48 45 42 39 35 32 29 26 23 1916 t (1) 1 62 59 56 53 49 46 43 40 37 33 30 27 24 21 17 t (2) 2 63 60 5754 50 47 44 41 38 34 31 28 25 22 18 t (3) 3 0 61 58 55 51 48 45 42 39 3532 29 26 23 19 t (4) 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 t (5) 5 1 62 59 5653 49 46 43 40 37 33 30 27 24 21 t (6) 6 2 63 60 57 54 50 47 44 41 38 3431 28 25 22 t (7) 7 3 0 61 58 55 51 48 45 42 39 35 32 29 26 23 t (8) 8 51 62 59 56 53 49 46 43 40 37 33 30 27 24 t (9) 9 6 2 63 60 57 54 50 4744 41 38 34 31 28 25 t (10) 10 7 3 0 61 58 55 51 48 45 42 39 35 32 29 26t (11) 11 8 5 1 62 59 56 53 49 46 43 40 37 33 30 27 t (12) 12 9 6 2 6360 57 54 50 47 44 41 38 34 31 28 t (13) 13 10 7 3 0 61 58 55 51 48 45 4239 35 32 29 t (14) 14 11 8 5 1 62 59 56 53 49 46 43 40 37 33 30 t (15)15 12 9 6 2 63 60 57 54 50 47 44 41 38 34 31 t (16) 16 13 10 7 3 0 61 5855 51 48 45 42 39 35 32 t (17) 17 14 11 8 5 1 62 59 56 53 49 46 43 40 3733 t (18) 18 15 12 9 6 2 63 60 57 54 50 47 44 41 38 34 t (19) 19 16 1310 7 3 0 61 58 55 51 48 45 42 39 35 t (20) 20 20 20 20 20 20 20 20 20 2020 20 20 20 20 20 t (21) 21 17 14 11 8 5 1 62 59 56 53 49 46 43 40 37 t(22) 22 18 15 12 9 6 2 63 60 57 54 50 47 44 41 38 t (23) 23 19 16 13 107 3 0 61 58 55 51 48 45 42 39 t (24) 24 21 17 14 11 8 5 1 62 59 56 53 4946 43 40 t (25) 25 22 18 15 12 9 6 2 63 60 57 54 50 47 44 41 t (26) 2623 19 16 13 10 7 3 0 61 58 55 51 48 45 42 t (27) 27 24 21 17 14 11 8 5 162 59 56 53 49 46 43 t (28) 28 25 22 18 15 12 9 6 2 63 60 57 54 50 47 44t (29) 29 26 23 19 16 13 10 7 3 0 61 58 55 51 48 45 t (30) 30 27 24 2117 14 11 8 5 1 62 59 56 53 49 46 t (31) 31 28 25 22 18 15 12 9 6 2 63 6057 54 50 47 t (32) 32 29 26 23 19 16 13 10 7 3 0 61 58 55 51 48 t (33)33 30 27 24 21 17 14 11 8 5 1 62 59 56 53 49 t (34) 34 31 28 25 22 18 1512 9 6 2 63 60 57 54 50 t (35) 35 32 29 26 23 19 16 13 10 7 3 0 61 58 5551 t (36) 36 36 36 36 36 36 36 36 36 36 36 36 36 36 36 36 t (37) 37 3330 27 24 21 17 14 11 8 5 1 62 59 56 53 t (38) 38 34 31 28 25 22 18 15 129 6 2 63 60 57 54 t (39) 39 35 32 29 26 23 19 16 13 10 7 3 0 61 58 55 t(40) 40 37 33 30 27 24 21 17 14 11 8 5 1 62 59 56 t (41) 41 38 34 31 2825 22 18 15 12 9 6 2 63 60 57 t (42) 42 39 35 32 29 26 23 19 16 13 10 73 0 61 58 t (43) 43 40 37 33 30 27 24 21 17 14 11 8 5 1 62 59 t (44) 4441 38 34 31 28 25 22 18 15 12 9 6 2 63 60 t (45) 45 42 39 35 32 29 26 2319 16 13 10 7 3 0 61 t (46) 46 43 40 37 33 30 27 24 21 17 14 11 8 5 1 62t (47) 47 44 41 38 34 31 28 25 22 18 15 12 9 6 2 63 t (48) 48 45 42 3935 32 29 26 23 19 16 13 10 7 3 0 t (49) 49 46 43 40 37 33 30 27 24 21 1714 11 8 5 1 t (50) 50 47 44 41 38 34 31 28 25 22 18 15 12 9 6 2 t (51)51 48 45 42 39 35 32 29 26 23 19 16 13 10 7 3 t (52) 52 52 52 52 52 5252 52 52 52 52 52 52 52 52 52 t (53) 53 49 46 43 40 37 33 30 27 24 21 1714 11 8 5 t (54) 54 50 47 44 41 38 34 31 28 25 22 18 15 12 9 6 t (55) 5551 48 45 42 39 35 32 29 26 23 19 16 13 10 7 t (56) 56 53 49 46 43 40 3733 30 27 24 21 17 14 11 8 t (57) 57 54 50 47 44 41 38 34 31 28 25 22 1815 12 9 t (58) 58 55 51 48 45 42 39 35 32 29 26 23 19 16 13 10 t (59) 5956 53 49 46 43 40 37 33 30 27 24 21 17 14 11 t (60) 60 57 54 50 47 44 4138 34 31 28 25 22 18 15 12 t (61) 61 58 55 51 48 45 42 39 35 32 29 26 2319 16 13 t (62) 62 59 56 53 49 46 43 40 37 33 30 27 24 21 17 14 t (63)63 60 57 54 50 47 44 41 38 34 31 28 25 22 18 15

An FHSS hop sequence typically distributes communications over a largepart of the frequency spectrum. FCC rules specify that equal channelutilization must be achieved with an ISM based system. In oneembodiment, hopping sequences may be uniquely defined or based ondifferent offsets into one pseudo-random sequence. All the networkdevices (regardless of hopping sequence) may be synchronized to the samenetwork time, and as illustrated by example in Table 1, the time slotsor steps, used for the acquisition channels, are common for all hopsequences. The data channels may be randomized, and for a given timeperiod, or network time step, i.e. t(x), each hopping sequence uses aunique data channel. In certain embodiments, this may only be the caseif there are fewer hopping sequences than data channels.

An FHSS implementation means that endpoint devices listen on theirassigned channel in each specific time slot. If an endpoint needs totransmit to a neighbor, it will transmit on the channel that thereceiving endpoint is listening. Consequently, a transmitting unit mustknow the frequency hopping sequence of the neighbor in order tocommunicate. For devices that are time synchronized and nottransmitting, the device may listen (i.e. in a receive mode) on thechannel defined for that specific time for that specific hoppingsequence. For example, referring to Table 1, at the “t(9)” time step adevice assigned to hopping sequence “7” listens on channel “50.” Fordevices that are time synchronized and transmitting, the transmittingdevice knows the hopping sequence of the destination (next hop) deviceand transmits on the proper channel for the receiving device. Immediateacknowledgements may be sent on the same channel as the receivedmessage. In an embodiment, the transmitting device may stay on the samechannel when waiting for an immediate response.

As mentioned above, in accordance with one aspect disclosed herein, theacquisition channel(s) may be utilized to expedite special networkcommunications within the mesh network. These communications may includesynchronization commands, network registration, neighbor discovery andre-synch after outage. Devices that are not yet time synchronized may bein a passive mode or an active mode. In an embodiment, devices notsynchronized are required to be in a passive mode. In a passive mode,the device may stay in a receive mode on one of the acquisition channelsfor a predetermined amount of time, which represents a timeout period.If during this timeout period, the device does not receive a networktime on the current acquisition channel, the device may switch to adifferent acquisition channel. In one embodiment, the time periodbetween changing acquisition channels may be directly related to theperiod between time broadcasts. In the active mode, a device mayperiodically transmit on one or more acquisition channels attempting tolocate a device or network.

Acquisition channels may be distributed across the available frequencyband. As mentioned, acquisition channels may be used to broadcastmessages, to send time synchronization messages, to perform neighbordiscovery, or during initial network formation. In other embodiments,acquisition channels may serve other functions.

Neighbor discovery is a process by which a network device (e.g., meter114 or collector 116) is able to discover the existence of other deviceswithin its communication range. In a mesh network of meters, neighbordevices may function as intermediate nodes on a communication pathbetween a meter and the collector on its LAN/subnet. Although neighbordiscovery may be performed on data channels, the use of acquisitionchannels as described herein may be advantageous. A goal of neighbordiscovery is for a device to discover the neighbor devices in itscommunication range with which the device can communicate with at a highlevel of performance. Neighbor discovery may occur for both synchronizedand unsynchronized devices. In order to communicate with a neighbordevice, a neighbor table may be employed. In one embodiment, a neighbortable may be maintained by each network device and may include, for eachdiscovered neighbor, an identity of the neighbor device, such as forexample an 8-byte MAC address and an indication of the frequency hoppingsequence used by that neighbor. During neighbor discovery, two neighborswill exchange this information so that each can store it in itsrespective neighbor table. In one embodiment, in addition to an 8-byteMAC address, a device could also store an optional 2-byte short addressfor a given neighbor, which short address could be assigned by anothernetwork entity, such as a collector 116 or an LBR.

Once neighbors are discovered, they may be used for routing messages inthe network. By knowing both the network time and the hopping sequenceof the destination (next hop) device, a transmitting device candetermine the correct channel for the communication.

As further mentioned above, network formation is another form of networkcommunications within a mesh network that may utilize the acquisitionchannels described herein. Devices that are not “connected” to thenetwork may use acquisition channels to join the network. In anembodiment, the collector 116 can initiate network formation by issuinga scan of nodes using an acquisition channel. When each node replies, itcan be provided with a time synchronization message to synchronize itsinternal clock, and each node can provide the collector 116 with itsrespective hop sequence. The collector 116 may attach to each of thenodes and may communicate with each device by using the appropriate hopfrequency. At a later time, the next acquisition channel can be utilizedto reach unregistered devices that might be listening on thatacquisition channel. Again, the collector 116 may synchronize theunregistered devices and receive the appropriate hop sequences forfurther communications. After the first level nodes are registered,these nodes can then proceed in a similar manner to synchronize andregister second hop devices.

Another form of network communication that may be performed on theacquisition channels is broadcast messaging. In one embodiment, whenbroadcasting a message, the start of the broadcast, e.g. at a LoWPANBorder Router (LBR), may be sent on the next acquisition channel withinthe frequency hopping sequences. In an embodiment, high prioritybroadcast messages may be sent on the next acquisition channel. Thestart of the broadcast may also be sent on a randomly selectedacquisition channel from the next brdcstAcqSlots, where brdcstAcqSlotsmay be assigned for a network or for each message. In an embodiment,broadcasting on a randomly selected acquisition channel is used for lowpriority messages. After the message has been broadcasted, thebroadcasted message may propagate through the network with each devicere-sending, or re-broadcasting, on the acquisition channel. Devicesreceiving the broadcast may re-broadcast in the same dwell period ifthey can obtain a clear channel. If a device cannot obtain a clearchannel prior to the end of the acquisition channel dwell time, thedevice may broadcast the message in the next acquisition channel dwelltime.

Network time synchronization may also be performed using the acquisitionchannels. Devices that are not yet time synchronized to the networktime—which may be maintained, for example, by a collector 116 via itsnetwork communications with a utility head-end-may receivesynchronization messages from a collector 116 on the acquisitionchannels. It should be appreciated that synchronized devices may alsoreceive time synchronization messages. In a synchronous frequencyhopping network, time synchronization across all devices is desirable.For example, assume time accuracy that exists between devices that haveclock crystals with 10 ppm accuracy. For these endpoint devices, in aone hour period of time, time may “slip” between devices as much as 36msec. If a frequency hopping sequence uses a dwell time of 400 msec,then 36 msec slip represents a 9% error.

FIG. 3 is an illustration of an embodiment of a time re-synchronizationprocess 300 that may be performed by a device in the network, such as ameter 114, when the meter 114 is in a passive mode. At step 302, thenetwork device tunes to an acquisition channel. At step 304, the networkdevice checks whether it has received a time synchronization message.This step may be performed, for example, after a time synchronizationmessage has been broadcasted. The broadcast may be transmitted byanother network device, such as meter 114 or collector 116, or by anetwork management system. In one embodiment, the time synchronizationmessage may be broadcasted on a plurality of the acquisition channels ineach frequency hopping sequence employed in the network. If a timesynchronization message is determined to have been received, then atstep 306, the device is synchronized to the network time indicated inthe received time synchronization message.

A time synchronization message contains information representing thecurrent time as determined and maintained by a network entity, such asby a head-end system operated by a utility. In one embodiment, acollector (e.g., collector 116) may receive absolute time from a masterstation of the utility periodically, and an absolute time may betransmitted to the other network devices in a time synchronizationmessage once per day. But in other embodiments, a time synchronizationmessage may comprise a value representing a number of fixed intervals(i.e. N×5 msec) from a certain pre-determined time, such as midnight, tominimize the amount of space taken in the packet of the timesynchronization message. If a network device is registering to thecollector for the first time, it should receive the absolute time. Inthe different hop levels of the network, the communication packet timepreferably is accounted for and adjusted within the next packet. When anetwork device receives a time synchronization message, it extracts thenetwork time from the message and may compare it to the time valuemaintained by its internal clock circuitry to determine whether thedevice is in sync. If the device is not in sync, then the device mayreplace its internal clock circuit time value with the received networktime value to synchronize to the network time. For example, the clockcircuitry may comprise a counter, and the device may replace its currentcounter value with the value representing the network time received inthe time synchronization message.

At step 308, after the device has been time synchronized, the device mayre-broadcast the time synchronization message. The re-broadcast mayeither be performed on the channel in which the broadcasted message wasreceived, or if the signal is not clear, then it may be re-broadcastedon the next clear acquisition channel. Thereafter, at step 310, thedevice hops from channel to channel according to its assigned frequencyhopping sequence.

Returning to step 304, if a broadcasted time synchronization message hasnot been received, then, at step 312, it is determined whether thedevice is within a timeout period. The timeout period represents apredetermined amount of time in which the device will remain on a givenacquisition channel. If the device is within the timeout period, thedevice continues to listen for a time synchronization message on itscurrent acquisition channel. If the timeout period expires withoutreceipt of a time synchronization message, then the device may be tunedto a different acquisition channel, where it begins listening on the newacquisition channel for a time synchronization message. This process maycontinue until a time synchronization message is received by the networkdevice.

In one embodiment, time synchronization messages are periodicallytransmitted by a network entity, such as a collector 116 or otherdevice, on the acquisition channel(s). In one alternative embodiment,the time synchronization messages may be transmitted by a LoWPAN BorderRouter (LBR). For example, in the embodiment described above in whichthere are four ACQ channels, each y times through a frequency hoppingsequence, the network entity may select one of the four acquisitionchannels and broadcast a time synchronization message. Devices receivingthe broadcast may re-broadcast in the same dwell period if they canobtain a clear channel. If a device cannot obtain a clear channel priorto the end of the current acquisition channel dwell time, it may attemptto re-broadcast the time synchronization message in the next acquisitionchannel dwell time. By using acquisition channels for this purpose, bothalready synchronized devices (which may or may not require an updatesbased on the transmitted network time) and unsynchronized devicesreceive time synchronization messages in a reasonably short amount oftime.

In another embodiment, time synchronization message broadcasts may beconfigured to occur on one acquisition channel for each frequencyhopping sequence period. In the example scenario described above, thesetime broadcasts would occur every 25.6 seconds (i.e., once every timethrough a complete hopping sequence). By way of further example, thetimeout period during which an unsynchronized device may listen on acurrent acquisition channel before moving to a next acquisition channel(e.g., in step 312 of FIG. 3) may be 4*25.6 seconds=102.4 seconds. Ifafter that amount of time a time synchronization message has not beenreceived, the device may tune to a next acquisition channel. In variousembodiments, the duty cycle of time synchronization broadcast messagesmay be increased to reduce synchronization time.

While the disclosure is described herein using a limited number ofembodiments, these specific embodiments are for illustrative purposesand are not intended to limit the scope of the disclosure as otherwisedescribed and claimed herein. Modification and variations from thedescribed embodiments exist. The scope of the invention is defined bythe appended claims.

What is claimed is:
 1. A communication system comprising at least afirst network device and a second network device, each network deviceincluding a transceiver; the first network device operating inaccordance with a first frequency hopping sequence, wherein the firstfrequency hopping sequence includes a plurality of data channels and atleast one acquisition channel; and the second network device operatingin accordance with a second frequency hopping sequence, wherein thesecond frequency hopping sequence includes a plurality of data channelsand the at least one acquisition channel, wherein the second frequencyhopping sequence is different from the first frequency hopping sequence,and wherein the at least one acquisition channel of the first frequencyhopping sequence and the at least one acquisition channel of the secondfrequency hopping sequence are arranged in their respective frequencyhopping sequences such that when the second network device is tuned tothe at least one acquisition channel of the second frequency hoppingsequence, the first network device will be tuned to the at least oneacquisition channel of the first frequency hopping sequence, and thefrequency of the at least one acquisition channel in each of the firstand the second frequency hopping sequences is the same.
 2. The system ofclaim 1, wherein the transceiver of the first network device isconfigured to tune to the at least one acquisition channel of the firstfrequency hopping sequence and receive thereon a time synchronizationmessage, and wherein a clock of the first network device is configuredto synchronize the first network device to a network time based on thetime synchronization message.
 3. The system of claim 2, wherein thefirst network device is configured to hop from one frequency to anotherfrequency according to the first frequency hopping sequence after theclock of the first network device is synchronized with the network time.4. The system of claim 2, wherein the first network device is configuredto broadcast the time synchronization message on the at least oneacquisition channel.
 5. The system of claim 4, wherein the secondnetwork device is further configured to re-broadcast the timesynchronization message on the at least one acquisition channel.
 6. Thesystem of claim 2, wherein the transceiver of the second network deviceis configured to tune to the at least one acquisition channel of thesecond frequency hopping sequence and receive thereon the timesynchronization message, and wherein a clock of the second networkdevice is configured to synchronize the second network device to thenetwork time based on the time synchronization message.
 7. The system ofclaim 2, further comprising: at least one other network device operatingin accordance with an assigned frequency hopping sequence, wherein theat least one other network device is in a receive mode on one of the atleast one acquisition channels until the time synchronization message isreceived, after the time synchronization message is received, the atleast one other network device hops from one frequency to anotherfrequency according to the assigned frequency hopping sequence.
 8. Thesystem of claim 1, wherein the first frequency hopping sequence and thesecond frequency hopping sequence are defined at the time of manufactureof the first network device and the second network device, respectively.9. The system of claim 1, wherein the first network device is configuredto initiate network formation by issuing a scan of each network deviceusing the at least one acquisition channel, wherein the first networkdevice is further configured to provide each network device using the atleast one acquisition channel with a time synchronization message tosynchronize their respective clocks.
 10. The system of claim 1, whereinthe first network device is configured to discover neighboring networkdevices, and wherein the first network device is capable ofcommunicating with each neighboring network device.
 11. The system ofclaim 10, the first network device is further configured to store, in amemory, information related to the frequency hopping sequence of eachneighboring device.
 12. A method for communication comprising: assigninga first frequency hopping sequence to a first network device, whereinthe first frequency hopping sequence includes a plurality of datachannels and at least one acquisition channel; and assigning a secondfrequency hopping sequence to a second network device, wherein thesecond frequency hopping sequence includes a plurality of data channelsand the at least one acquisition channel, wherein the second frequencyhopping sequence is different from the first frequency hopping sequence,and wherein the at least one acquisition channel of the first frequencyhopping sequence and the at least one acquisition channel of the secondfrequency hopping sequence are arranged in their respective frequencyhopping sequences such that when the second network device is tuned tothe at least one acquisition channel of the second frequency hoppingsequence, the first network device will be tuned to the at least oneacquisition channel of the first frequency hopping sequence, and thefrequency of the at least one acquisition channel in each of the firstand the second frequency hopping sequences is the same.
 13. The methodof claim 12, further comprising: tuning, by a transceiver of the firstnetwork device, to the at least one acquisition channel of the firstfrequency hopping sequence; receiving thereon a time synchronizationmessage; and synchronizing, by a clock of the first network device, thefirst network device to a network time based on the time synchronizationmessage.
 14. The method of claim 13, further comprising: hopping, by thefirst network device, from one frequency to another frequency accordingto the first frequency hopping sequence after the clock of the firstnetwork device is synchronized to the network time.
 15. The method ofclaim 12, further comprising: assigning a frequency hopping sequence toat least one other network device; and adding the at least one othernetwork device to the frequency hopping network, wherein the at leastone other network device is in a receive mode on one of the at least oneacquisition channels until a time synchronization message is received,after the time synchronization message is received, the at least oneother network device hops from one frequency to another frequencyaccording to the assigned frequency hopping sequence.
 16. The method ofclaim 12, further comprising: discovering, by the first network device,neighboring network devices, wherein the first network device is capableof communicating with each neighboring network device.
 17. The method ofclaim 16, further comprising: storing, in a memory of the first networkdevice, information related to the frequency hopping sequence of eachneighboring network device.
 18. The system of claim 12, furthercomprising: issuing a scan of each network device, by the first networkdevice, using the at least one acquisition channel; and forming, by thefirst network device, a frequency hopping network based on the scan ofeach network device.
 19. The system of claim 18, further comprising:receiving, by the first network device, a reply to the issued scan fromeach network device; and initiating, by the first network device, theformation of the frequency hopping network based on the reply to theissued scan.